Update dependency bignumber.js to v9

[renovate-bot]

This PR contains the following updates:

Package	Type	Update	Change
bignumber.js	devDependencies	major	8.1.1 -> 9.0.0

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Release Notes

MikeMcl/bignumber.js

### [`v9.0.0`](https://togithub.com/MikeMcl/bignumber.js/blob/master/CHANGELOG.md#​900) [Compare Source](https://togithub.com/MikeMcl/bignumber.js/compare/v8.1.1...v9.0.0) ##### 9.0.0 - 27/05/2019 - For compatibility with legacy browsers, remove `Symbol` references.

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